Pulsatile administration of estradiol effectively primes orchidectomized ( O R C H ) male guinea pigs to display progesterone-facilitated lordosis. In contrast, a single injection of estradiol benzoate ( E B ) is not behaviorally effective. In ovariectomized female guinea pigs, estradiol pulses induce progestin receptors selectively in substance P neurons in the ventrolateral hypothalamus (VLH), a site at which estradiol primes females to respond behaviorally to progesterone. T o test the hypothesis that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P neurons in the VLH in males, ORCH animals received a single injection of EB 40 h before, or two pulses of estradiol-l7& 39 and 11 h before perfusion. Colchicine was administered intracerebroventricularly prior to perfusion. The only difference found between the two estradiol treatment groups was a higher number of progestin receptorimmunoreactive (PR-IR) cells in the rostra1 V L H of es-tradiol pulse-treated males. There were no significant differences in the number of PR-IR cells in the mid-or caudal VLH, nor in the number of substance P-immunoreactive (SP-IR) neurons in the VLHlventromedial hypothalamus (VMH) of animals receiving the two estradiol treatments. Furthermore, the percentage of PR-IR cells in the VLH also immunoreactive for SP did not differ between the estradiol pulse-(22'7~25%) and the EB-injected animals (22%-32%). These data do not support the hypothesis that administration of behaviorally effective estradiol pulses, as compared to behaviorally ineffective EB injections, induce progestin receptors selectively in substance P neurons in the VLH of male guinea pigs.